I have found info and solutions about it on tmux's pages:
Screen's terminfo description lacks italics mode and has standout mode in its place, but using the same escape sequence that
urxvt uses for italics. This means applications (like
vim) looking for italics will not find it and might turn to reverse in its place, while applications (like less) asking for standout will end up with italics instead of reverse.
[Fix 1] Replace terminfo entries and get reverse + italics
To make applications aware that
tmux supports italics and to use a proper escape sequence for standout, you'll need to create a new terminfo file with modified
$ mkdir $HOME/.terminfo/
$ infocmp "$screen_terminfo" | sed \
-e 's/^screen[^|]*|[^,]*,/screen-it|screen with italics support,/' \
-e 's/%?%p1%t;3%/%?%p1%t;7%/' \
-e 's/smso=[^,]*,/smso=\\E[7m,/' \
-e 's/rmso=[^,]*,/rmso=\\E[27m,/' \
-e '$s/$/ sitm=\\E[3m, ritm=\\E[23m,/' > /tmp/screen.terminfo
$ tic /tmp/screen.terminfo
And tell tmux to use it in ~/.tmux.conf:
set -g default-terminal "screen-it"
If your terminal supports 256 colors, use:
instead of "screen". See the FAQ entry about 256 colors support for more info. Also note that tmux will still display reverse video on terminals that do not support italics.
urxvt cannot display italics at all, make sure you have an italics capable font enabled, for example, add to ~/.Xdefaults:
urxvt.italicFont: xft:Bitstream Vera Sans Mono:italic:autohint=true
[Fix 2] Simply replace italic control characters with reverse
Add this terminal override to ~/.tmux.conf:
set -g terminal-overrides 'rxvt-unicode*:sitm@,ritm@'